The Access Network Discovery and Selection Function is an entity introduced by 3GPP as part of their Release 8 set of specifications, within an Evolved Packet Core of the System Architecture Evolution for 3GPP-compliant mobile networks. The purpose of the ANDSF is to assist a UE to discover non-3GPP access networks, such as WLAN or WIMAX, that can be used for data communications in addition to 3GPP access networks (such as HSPA or LTE) and to provide the UE with rules policing the connection to these networks. The ANDSF is described, for example, in Chapter 4.8 of the 3GPP Specification TS 23.402, v.10.4.0, Release 10 (June 2011). For convenience, an overview of the key features of the ANDSF is provided in the following.
An ANDSF provides one or more of the following kinds of information to a UE, based on operator configuration:    (1) Inter-System Mobility Policy (ISMP), comprising a set of operator-defined network selections rules for a UE with no more than one active access network connection (for example, either LTE or WLAN); in other words, a UE that can route IP traffic only over a single radio access interface at a given time. The ISMP can be used by the UE to determine when inter-system mobility is allowed or restricted (e.g. from one access technology type to another, such as a handover from WLAN to WiMAX, or when certain conditions are met). The ISMP can also identify which technology type or which access network is most preferable for accessing the EPC, for example, if a specific access technology type is preferable to another (e.g. WiMAX is preferable to WLAN) or if a specific access network identifier is preferable to another (e.g. WLAN SSID-1 is preferable to WLAN SSID-2);    (2) Inter-System Routing Policy (ISRP), comprising network selection rules for a UE with potentially more than one active access network connection (e.g. both LTE and WLAN). Such a UE may employ IP Flow Mobility (IFOM), Multiple Access PDN Connectivity (MAPCON) or non-seamless WLAN offload according to operator policy and user preferences; and    (3) Access Network Discovery Information (ANDI), comprising a list of networks that may be available in the vicinity of the UE, for all access technology types requested by the UE (if any are requested) and information assisting the UE to expedite the connection to these networks. The ANDI identifies the access technology type of an available access network (e.g. WLAN or WiMAX), the radio access network identifier (e.g. the SSID of a WLAN), as well as other technology-specific information, such as one or more carrier frequencies which the UE is to use. The ANDSF is able to limit the amount of ANDI provided to the UP based on the UE's geographical location and its technological capabilities, i.e. the access technology types with which it is compatible.
The ANDSF communicates with the UE over the S14 reference point, which is essentially a synchronization of an OMA-DM Management Object (MO) specific to ANDSF. Details of the ANDSF MO are disclosed in 3GPP Specification TS 24.312, v.10.3.0, Release 10 (June 2011).
A shortcoming of existing ANDSF implementations is that they are not optimised with respect to the delay that occurs between the transmission of ANDI being triggered (either by the ANDSF receiving a request from the UE or by the transmission being triggered by a network event) and the consequent implementation of procedures by the UE to discover and reselect a higher-priority access network where the received ANDI indicates that such an access network is available in the vicinity of the UE. It is therefore highly desirable to find a way of reducing this delay.